Control devices of the type indicated above, which work with a pressure medium such as hydraulic oil or compressed air, are known in various designs and are used with automated transmissions of motor vehicles to carry out gear changes. In passenger cars those control devices are usually hydraulic, whereas in contrast, in larger commercial vehicles such as trucks and buses, which have compressed air units, they mainly operate pneumatically.
Largely identical designs of such a control device are described for example in DE 199 31 973 A1 and in DE 101 31 853 A1. There, in each case a pump is provided by which a pressure medium can be drawn from a storage reservoir or oil sump and conveyed to a main pressure line. By means of a main shut-off valve, made as a 2/2-way magnetic switching valve, a pressure line can be selectively connected to, or disconnected from the main pressure line. To this pressure line are connected a plurality of control valves in the form of 3/2-way magnetic switching valves, which are associated in pairs with a respective actuating device. The actuating devices are each made as a double-action actuating cylinder with two pressure spaces separated by a piston, and the pressure spaces are in each case connected by a connection line to one of the associated control valves, by means of which they can be connected selectively to the pressure line or to an unpressurized line.
Depending on the structure of the transmission-internal shift actuating device, the actuating devices may have the function of a selector control element for selecting one among several shift gates, or of a gear control element for engaging and disengaging the gears of a shift gate concerned, or they may function exclusively as a gear control element. If the shift actuation is effected by an axially displaceable and rotatable shifting shaft, an actuating device that works as a selector control element is needed, by means of which, to select the shift gate, the shifting shaft can be manipulated into form-fitting engagement with the gearshift rod of the shift gate concerned, for example by means of a shift finger. Then, by virtue of another shift actuating device that acts as a shift control element, the associated gear is engaged and disengaged by the shifting shaft by axial displacement of the gearshift rod, which is engaged with an operating sleeve via a shifting fork.
It is also possible, however, for the gearshift rods or shift rockers to be actuated directly, in each case by an associated actuating device. In this case all the actuating devices act as gear control elements, and the shift gate is selected exclusively by actuating the gear control elements. In such a case the number of actuating devices needed corresponds to the number of shift gates, so in a simple automated shift transmission with six forward gears and one reverse gear at least four actuating devices are needed.
Starting from the last-mentioned example of a simple automated shift transmission with gearshift rods or shift rockers that can be actuated directly by the actuating devices, there are gearshifts which need a larger, and ones which need a smaller amount of pressure medium. In a gearshift between two gears associated with the same shift gate, i.e. which are engaged or disengaged by the same shifting rod or shift rocker, only one actuating device is used so the demand for pressure medium is only relatively small.
In contrast, if a shift takes place between two gears associated with different shift gates, i.e. which are engaged and disengaged by different shifting rods or shift rockers, then two actuating devices are used so the demand for pressure medium is greater. The pressure medium demand is even greater still in a so-termed multiple shift during which a plurality of successive gearshifts take place at short intervals.
The main shut-off valve is now required, during shift pauses, to cut off the pressure line with its connected control valves and the actuating devices from the main pressure line when in its closed condition. This then protects the control valves and actuating devices from the relatively high main pressure in the main pressure line, whereby otherwise possible leakage losses and perhaps also undesired movements of the actuating devices are avoided.
On the other hand, during shift phases the main shut-off valve is opened and the pressure line is therefore connected to the main pressure line in order to provide the control valves and the actuating devices to be operated by them with a sufficiently high pressure and a large enough volume flow. Consequently, the main shut-off valve is designed for a volume flow which, having regard to leaks that result from wear, corresponds to the maximum volume flow that can be required during a shift process. Thus, the main shut-off valve is usually of relatively large size and is consequently comparatively expensive and prone to malfunction; because of marked hysteresis its control properties are poor and when configured, as is usual, as a magnetic switching valve, it demands a relatively high control current. Furthermore, a malfunction of the main shut-off valve causes a failure of the entire control system and the associated shift transmission can no longer carry out gearshifts.
To avoid these disadvantages, at least in part, it is also known, instead of one large magnetic switching valve, to adopt a so-termed booster arrangement in which a correspondingly large, pressure-controlled main shut-off valve is positioned between the main pressure line and the pressure line, which can be acted upon with a control pressure via a smaller, interposed valve made as a magnetic switching valve and connected to the main pressure line. Owing to the rapid response behavior of and the low control current needed by this interposed valve, the controllability of this valve arrangement is at least partially improved. However, since two valves are needed this arrangement costs more and its operational reliability is to say the least not improved, since now only one defect in either one of the two valves, the pressure-controlled main shut-off valve or the electrically controlled interposed valve, can lead to failure of the control system.